Hard optical coating materials usually have a fixed refractive index. In order to obtain optical coatings with suitable refractive index to match different requirements, the prior art process is to search for a coating material with proper refractive index to suit a need. I know of no technique whereby one coating material can be used with adjustable refractive index to suit all needs.
I have discovered that a PLZT optical coating, which is a hard optical coating, can have its refractive index adjusted by diffusion to match a required value thereby allowing the use of a single material with adjustable refractive index for many varied optical coating needs.
PLZT ceramics with a Zr/Ti ratio in the range around 65/35 and with La doping around 8 - 9% have been heated in a controlled atmosphere up to 800.degree. C. The surface refractive index was measured and found to decrease monotonically from about 2.5 down to about 1.5 to 6328A wavelength, as a function of the number of heating cycles or total time lapse at the peak temperature. This refractive index variation is due to out-diffusion of one or more of the elements in PLZT. This fact combined with the finding that durable films of high refractive indices (.about.2.5) can be prepared by sputtering, makes PLZT a suitable material for optical coating. The refractive index of the coating can be adjusted by heating the film in a controlled atmosphere, in one case a vacuum of about 10.sup.-.sup.4 Torr, for a suitable time duration. As referred to herein, by optical coatings is meant coatings having a thickness of from about a quarter wavelength up to about one wavelength (.lambda.), i.e. in the range of about 0.1 to about 1 micron in thickness.